404 M. P. Riess on the Action of Non-conducting Bodies 



In order to give certain and unequivocal results, the experi- 

 ment in question requires some care when arranging the appa- 

 ratus, which I will consequently describe at length. Two 

 metallic discs, insulated on glass pillars, were placed vertically 

 on a table with their faces turned towards each other, and both 

 parallel to the edge of the table at which the experimenter sat. 

 The second or most distant plate was connected, by means of a 

 wire 18 inches long, with a gold-leaf electroscope; and in finer 

 experiments, with a dry-pile electroscope, and the first or nearest 

 disc was charged with positive electricity. The electroscope in- 

 dicated positive electricity, Avhich was removed by uninsulation. 

 If, during the course of the experiment, the quantity of induced 

 electricity on the disc in connexion with the electroscope was 

 increased, in other words, if the induction of the charged disc 

 was apparently strengthened, the electroscope again indicated 

 positive electricity ; on the contrary, if that induction was appa- 

 rently weakened, it indicated negative electricity. In order to 

 avoid all uncertainty about the result thus obtained, the electro- 

 scope was uninsulated after the change in the induced electricity, 

 and the indication of the electroscope observed after the cause of 

 this change had been removed : here an indication of positive 

 electricity proved that in the original experiment the induced 

 electricity had been diminished ; of negative electricity, that the 

 induction had been increased. After every experiment such a 

 check was applied, so that no doubt can be entertained as to the 

 correctness of the observations. At first two metallic discs, 7 - 21 

 inches in diameter and O06 inch thick, were used, whose distance 

 asunder was 1"]1 inch. A disc of shell-lac, 7"21 inches in dia- 

 meter and 033 inch thick, which had received a smooth reflect- 

 ing surface by pressure between two glass plates, was fixed by 

 means of a clamp to a glass pillar, 5 inches long ; the latter was 

 covered with shell-lac and turned on a point, so as to admit of 

 its being placed in any required position between the two metallic 

 discs. When the disc of shell-lac, after being divested of all 

 electricity by means of a spirit-lamp flame, was brought in the 

 middle between the two discs and made concentric with them, 

 the gold-leaf or dry-pile electroscope showed that the induced 

 electricity upon the disc in connexion with the electroscope was 

 considerably increased. On the contrary, when the disc of shell- 

 lac only occupied a portion of the intermediate space (a spindle- 

 shaped portion about half an inch in breadth), and its greatest 

 part remained without that space, the induced electricity was, 

 although only slightly, yet quite decidedly diminished. 1 men- 

 tion this, because whatever may be the constitution and shape of 

 the interposed disc, the electroscope always indicates a diminu- 

 tion in the amount of induced electricity at the moment when it 



